Speaking as someone who was an extremely heavy user of Usenet back in the day, it seems pretty clear that spammers overwhelmed it well before it became primarily used for binaries, and most people were happy to turn to web forums once they were available.

(Edit: moderated Usenet groups appeared fairly early, but what I recall is that spammers overwhelmed the moderators, even there.)

Usenet never completely died btw, so that's not a fair description; there are people using it as a forum to this day, in nontrivial absolute numbers, but of course small numbers relatively, compared with web forums.